Gamma cameras typically are used for locating and displaying abnormalities in human glands and organs. More specifically, and with respect to using a gamma camera, gamma-ray-emitting tracer material is administered to a patient, and the tracer material is more greatly absorbed by the abnormality to be detected than by the other tissues. The gamma camera generates data, or an image, representing the distribution of such tracer material within the patient.
A conventional gamma camera includes a collimator and a scintillation crystal, or detector, responsive to radiation stimuli, i.e., gamma rays emitted by the patient. The collimator is positioned adjacent one face of the crystal, and includes a collimator core fabricated from gamma ray attenuating material and having a plurality of openings. An array of photomultiplier tubes typically are positioned adjacent an opposite face of the crystal.
In operation, the gamma rays emitted by the patient are projected toward the collimator core, and those rays projecting through the collimator openings interact with the crystal. The gamma rays impinging upon the collimator septa, i.e., impinging upon the attenuating material and not projecting through the collimator openings, are substantially attenuated and do not interact with the crystal. Particularly, the collimator attenuates, or blocks, certain rays from reaching the crystal. For example, rays which travel at an angular orientation with respect to the collimator openings, i.e., rays which penetrate the collimator septa, may be completely blocked and may not impinge upon the crystal. By blocking these rays, the image quality is improved because such rays generally result in erroneous readings with respect to position and intensity.
Light events occur within the crystal at locations where the rays interact with the crystal lattice structure. The photomultiplier tubes, in response to the light events, produce individual analog outputs. In digital gamma cameras, the analog photomultiplier tube outputs are supplied to analog-to-digital converters (ADCs) which convert the analog outputs to digital signals.
To generate an image, a representation of the distribution of events in the crystal is generated by utilizing a matrix of storage registers whose elements are in one-to-one correspondence with elemental areas of the crystal. The crystal elemental areas are identified by coordinates. Each time a light event occurs in the crystal, the event coordinates are identified and the register in the storage register matrix corresponding to the identified event coordinates is incremented. The contents of a given register in the matrix is a number that represents the number of events that have occurred within a predetermined period of time within an elemental area of the crystal. Such number is directly proportional to the intensity of radiation emitted from an elemental area of the radiation field. The number stored in the register therefore is used to establish the brightness of a display picture element corresponding to the crystal elemental area. The distribution of a radiation field is displayed in terms of the brightness distribution of the display.
Gamma cameras may be used in connection with ultra-high energy isotopes or tracers such as F-18 and FDG. Such high energy isotopes and tracers may generate radiation having an energy value approaching 511 keV. At this energy level, known collimators typically are not effective in preventing unaligned gamma rays from impinging upon the scintillation crystal. Particularly, the higher energy gamma rays are known to penetrate the collimator septa and impinge upon the scintillation crystal, thus reducing image contrast and diagnostic image quality.
To reduce such undesirable collimator penetration caused by high energy isotopes, collimators have been modified to include thicker collimator cores. For example, a lead collimator having a half-value thickness of approximately 6.5 mm is believed to adequately prevent undesirable radiation from penetrating the collimator. Although such thicker collimators reduce gamma ray penetration from high energy isotopes, such thicker collimators also weigh substantially more than typical collimators. With this increased weight, the collimator may exceed the weight bearing capacity of the gamma camera or other nuclear imaging system components such as a collimator cart or exchange system. In addition, changing collimators for different imaging sessions is more cumbersome with heavier collimators.
Rather than utilizing a thicker collimator, reduced field of view collimators may be used to reduce undesired radiation penetration. One such reduced field of view collimator is described, for example, in U.S. patent application Ser. No. 08/853,279 (15-NZ-4480), entitled Gamma Camera Collimator, filed concurrently herewith and assigned to the present assignee. Such reduced field of view collimators are lighter than known collimators, and substantially confine gamma ray penetration to a small portion of the crystal face where imaging will occur. Accordingly, such collimators typically facilitate good imaging without exceeding the weight bearing limitations of nuclear gamma system mechanics, i.e., weight bearing limitations of a system gantry, when utilizing high energy isotopes.
Even with the collimator described above, gamma ray penetration though the collimator may cause a light event outside the desired field of view. Such a light event causes a responsive photomultiplier tube to produce an analog output and contribute to a camera count rate. A high count rate is undesirable in gamma cameras, because such a high count rate degrades image quality. Particularly, the camera electronics are sensitive to count rate, and a high count rate often causes substantial dead time, which is undesirable. In addition, a high count rate may cause an apparent loss of sensitivity since the camera cannot simultaneously record an event within the field of view and elsewhere.
It would be desirable to reduce dead time in gamma camera electronics associated with light events occurring outside of the camera field of view. It also would be desirable to substantially maintain camera sensitivity without significantly reducing image quality.